In general, in picking up a video image or a still image with a camera such as a 3-charge-coupled-device (3-CCD: three-plate type) camera or a single plate type CCD camera, there has been a problem of a significant decrease in image quality due to a halation and the like when picking up an image with great latitude (a range of difference between bright and dark portions) or a special image having an intense highlight portion.
For example, in the pickup of an image of a contrasty subject, as in video image pickup in outer space, if the sun is placed in a field of view and exposure is adjusted to the sun, the sun is only visible, whereas if the exposure is adjusted so that its surrounding portions are also visible, a halation is caused due to sunlight in the portions other than the sun, resulting in deterioration in the image quality. Moreover, when an image of a road or the like is picked up with a video camera at night, the headlights of automobiles alone are strongly visible, which optically and electronically affects other portions and leads to deterioration in the entire image quality.
Here, in cameras such as conventional cameras, video cameras and digital cameras, measures have been taken; gamma characteristics are transformed into a nonlinear form and saturated so as to electronically cut peak portions, thereby suppressing highlight (e.g., refer to Japanese Patent Publication Laid-open No. 5-244623 (pages 2 to 3, FIG. 7), Japanese Patent Publication Laid-open No. 6-23331.0 (page 2, FIG. 3) and Japanese Patent Publication Laid-open No. 8-149378 (page 2, FIG. 5)).
Such electronic processing can hold down the highlight to some degree, and it has been possible to acquire an image in which the highlight is held down in appearance.
However, according to such a conventional method, the electronic processing is performed after image light reaches an imaging plane, so that the imaging plane has already suffered optical adverse effects that deteriorate image quality, such as a halation caused by light scattering or a fog, and it has been impossible to remove these effects. Thus, the electronic processing in the conventional method alone has not led to a fundamental solution of the highlight image problem.
Furthermore, in general cameras, the sensitivity of the entire image is decided by the brightness of the highlight portion, so that the signal-to-noise ratio (S/N: a value serving as an index to evaluate image quality) in the dark portion significantly decreases, and there has been a problem of unavoidable excessive deterioration of the image quality in the dark portion.
Moreover, especially when an image of, for example, a road is picked up at night, the problem of a halation due to light scattering is caused especially around headlights even if a bright portion with headlights is saturated to adjust the exposure to a dark side, resulting in a decrease in the image quality.
There has been another problem wherein output is saturated in the portion corresponding to the headlights of automobiles or the like owing to the characteristics of imaging elements, and subsequent processing of an image signal once saturated only makes a slight improvement, such that an extreme decrease is inevitable in the amount of image information in the bright portion.
As described above, the methods heretofore proposed are capable of alleviating to some degree the adverse effects of optical scattering or the like caused by the headlights of the automobiles through electronic processing, but are incapable of essentially removing these adverse effects.
The present invention has been proposed to solve such problems in the conventional techniques, and is directed to provide to a highlight suppression image pickup apparatus wherein in an image with great latitude or an image having an intense highlight portion, the highlight components can be optically and electronically removed and suppressed before light reaches an imaging element, and the effects of the highlight are fundamentally and essentially suppressed and removed so that a high-quality image can be acquired while preserving as much information for the highlight portion as possible.